RegionLoader: specify type of unpack() return

PHPStan has no idea what is going on in this code because unpack() returns mixed[].
Possibly it might be a good idea to implement a dynamic return type extension for this.

src/pocketmine/level/format/io/region/RegionLoader.php

@@ -303,6 +303,7 @@ class RegionLoader{
 			throw new CorruptedRegionException("Corrupted region header (unexpected end of file)");
 		}
 
+		/** @var int[] $data */
 		$data = unpack("N*", $headerRaw);
 
 		for($i = 0; $i < 1024; ++$i){